Combustion-chamber structure of engine

ABSTRACT

A combustion-chamber structure of an engine comprises a combustion chamber which is partitioned by a cylinder block, a cylinder head, a piston, an intake valve, and an exhaust valve. The intake valve (exhaust valve) comprises an intake valve body including an umbrella part having a valve head and a valve face, a heat-insulation layer provided at the valve head and having smaller heat conductivity than the valve body, a heat-barrier layer provided to cover the valve head with the heat-insulation layer and having smaller heat conductivity than the valve body and the heat-insulation layer, and a heat-diffusion layer provided between the heat-insulation layer and the heat-barrier layer and having larger heat conductivity than the heat-insulation layer and the heat-barrier layer. The heat-diffusion layer comprises a contact portion which contacts with the cylinder head when the intake valve is closed.

BACKGROUND OF THE INVENTION

The present invention relates to a combustion-chamber structure of anengine which comprises a heat-barrier layer to suppress heat loss.

A combustion chamber of a gasoline engine or the like for a vehicle isrequired to reduce heat dissipation (heat loss) through a wall surfaceof the combustion chamber. A technology that a heat-barrier layer madeof a small heat-conductivity material is coated on thecombustion-chamber wall surface, such as a crown surface of a piston,for heat-loss reduction is known. A temperature difference betweencombustion gas generated in the combustion chamber and thecombustion-chamber wall surface is made so small by providing theheat-barrier layer that the heat loss can be reduced.

Japanese Patent Laid-Open Publication No. 2018-172997 discloses acombustion-chamber structure in which a heat-insulation layer isprovided at a piston crown surface in addition to the heat-barrierlayer. The heat-barrier layer covers an entire part of the piston crownsurface, thereby suppressing the heat dissipation through a piston body.The heat-insulation layer is provided below the heat-barrier layer andin a central area, in a radial direction, of the piston crown surface,thereby making this central area be the area where the heat does notescape easily. Thereby, a temperature distribution in which thetemperature of an central area, in a radial direction, of the combustionchamber is relatively high, whereas the temperature of an outside area,in the radial direction, of the combustion chamber is relatively low isformed. This temperature distribution has a merit that in a case where ahomogeneous-charge compression-ignition combustion (in other words, apremixed compression-ignition combustion) is performed, the combustionis made properly slow and thereby a rapid increase of a cylinderinternal pressure or heat loss can be properly suppressed.

The combustion chamber is also partitioned by an intake valve and anexhaust valve. Accordingly, it is necessary to suppress the heatdissipation from the intake valve and the exhaust valve as well forreduction of the heat loss of the combustion chamber. Herein, it may beconsidered that the heat-barrier layer and the heat-insulation layer arealso provided at respective valve heads of the intake valve and theexhaust valve, similarly to the structure of the above-described patentdocument. There is a problem, however, that the heat may be excessivelystored at the heat-insulation layer, thereby making the temperature ofthe valves improperly high. That is, the heat-insulation layer may storethe heat which has not been insulated (blocked) by the heat-barrierlayer, so that this heat-insulation layer having the high temperaturemay heat the heat-barrier layer. This heating may cause a temperatureincrease of the valve itself, thereby increasing the cylindertemperature. Thereby, the air taken in an intake stroke of the enginemay be heated excessively, so that improper preignition may occur in acompression stroke of the engine.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a combustion-chamberstructure of an engine which can properly suppress the temperatureincrease of the valve which may cause the preignition, attaining theheat-loss reduction.

A combustion-chamber structure of an engine according to the presentinvention comprises a cylinder block, a cylinder head, a piston, avalve, and a combustion chamber partitioned by the cylinder block, thecylinder head, the piston, and the valve, wherein the valve isconfigured to open or close a port opening to the combustion chamber andcomprises an umbrella part and a stem part, the umbrella part of thevalve includes a valve body which includes a valve head facing thecombustion chamber and a valve face positioned on an opposite side tothe combustion chamber, a heat-insulation layer which is provided at thevalve head and has smaller heat conductivity than the valve body, aheat-barrier layer which is provided to cover the valve head providedwith the heat-insulation layer and has smaller heat conductivity thanthe valve body and the heat-insulation layer, and a heat-diffusion layerwhich is provided between the heat-insulation layer and the heat-barrierlayer and has larger heat conductivity than the heat-insulation layerand the heat-barrier layer, and the heat-diffusion layer comprises acontact portion which is provided to extend up a position of theumbrella part of the valve which contacts with the cylinder head whenthe valve is closed.

According to the present combustion-chamber structure, the valve head iscovered with the heat-barrier layer having the smaller heat conductivitythan the valve body and the heat-insulation layer. Accordingly, thetemperature difference between the valve head and the combustion chamberis made so small that heat transfer to the valve body can be suppressed.Further, the heat which has passed through the heat-barrier layer isstored at the heat-insulation layer. Accordingly, the high temperatureof the heat-barrier layer (valve head) can be maintained. Meanwhile, theheat-diffusion layer is provided between the heat-insulation layer andthe heat-barrier layer. This heat-diffusion layer has the larger heatconductivity than both the heat-barrier layer and the heat-insulationlayer and comprises the contact portion contacting with the cylinderhead. Accordingly, even in a case where the heat-insulation layer hasstored the heat excessively, this heat can be made to escape to thecylinder head through the heat-diffusion layer. Consequently, thetemperature increase of the valve which may cause the preignition can beprevented properly.

In the above-described combustion-chamber structure of the engine, it ispreferable that the cylinder head have the larger heat conductivity thanthe valve body.

According to this combustion-chamber structure, the heat of theheat-insulation layer which is transferred through can be made to escapeto the cylinder head more than the valve body.

In the above-described combustion-chamber structure of the engine, it ispreferable that the cylinder head comprise a valve seat which isprovided at the port opening and with which a portion of the umbrellapart of the valve body contacts, and the contact portion of theheat-diffusion layer be provided at the portion of the umbrella partwhich contacts with the valve seat.

The valve seat provided at the port opening necessarily contacts withthe umbrella part of the valve when the intake or exhaust port isclosed. Accordingly, a heat dissipation path (route) from the valve bodyto the cylinder head can be secured by providing the contact portion ofthe heat-diffusion layer at the portion of the umbrella part whichcontacts with the valve seat.

In the above-described combustion-chamber structure of the engine, it ispreferable that the valve be an intake valve, and the heat-barrier layerbe provided on the valve face of the umbrella part of the valve as well.In this case, it is preferable that the heat-barrier layer be providedon the stem part of the valve as well.

According to this combustion-chamber structure, the heat dissipationfrom the valve face or the stem part of the valve can be suppressed bythe heat-barrier layer even in a case where the temperature of the valvebody of the intake valve increases. Accordingly, the air passing throughthe intake port is suppressed from being heated excessively by theintake valve, so that the preignition can be prevented properly.

In the above-described combustion-chamber structure of the engine, it ispreferable that the valve be an exhaust valve, and the heat-barrierlayer be provided on the valve face of the umbrella part of the valve aswell. In this case, it is preferable that the heat-barrier layer beprovided on the stem part of the valve as well.

According to this combustion-chamber structure, the surface temperatureof the valve face of the umbrella part and the stem part of the exhaustvalve can be maintained at the high temperature by the heat-barrierlayer. The exhaust valve provided at the exhaust port is exposed to hightemperature by exhaust heat of the combustion gas. Accordingly, the heattransfer to the valve body of the exhaust valve, i.e., the heat loss,can be suppressed properly by providing the heat-barrier layer on thevalve face and the stem part of the exhaust valve.

In the above-described combustion-chamber structure of the engine, it ispreferable that the heat-diffusion layer include a first portion whichis provided between the heat-insulation layer and the heat-barrier layerat the valve head, the contact portion, and a second portion which is anunderlayer of the heat-diffusion layer which is provided at the valveface and the stem part.

According to this combustion-chamber structure, the first portion of theheat-diffusion layer receives the heat of the valve head and the secondportion receives the heat of the valve face and the stem part of thevalve. The heat received by the first portion and the second portion ofthe heat-diffusion layer is made to escape from the contact portion tothe cylinder head. The exhaust valve receives the heat from exhaust gaspassing through the exhaust port, so that its temperature increases.Meanwhile, the intake valve receives the heat from EGR gas or blow-backgas of the combustion gas from the combustion chamber which is caused bysetting a valve overlap term, so that its temperature possiblyincreases. Accordingly, by configuring the heat-diffusion layer tocomprise the above-described first portion and the above-describedsecond portion, the excessive temperature increase of the exhaust valveand the intake valve can be prevented properly.

In the above-described combustion-chamber structure of the engine, it ispreferable that the valve be an exhaust valve, the heat-insulation layerand the heat-diffusion layer be provided to cover an entire part of theumbrella part of the valve, and the heat-barrier layer be provided tocover the entire part of the umbrella part of the valve except thecontact portion of the heat-diffusion layer. In this case, it ispreferable that the heat-insulation layer, the heat-diffusion layer, andthe het-barrier layer be provided to cover at least a section of thestem part of the valve which is continuous to the umbrella part of thevalve.

According to this combustion-chamber structure, the umbrella part of theexhaust valve is covered with three layers of the heat-insulation layer,the heat-diffusion layer, and the heat-barrier layer except theabove-described contact portion. More preferably, at least a portion ofthe stem part which is continuous to the umbrella part is covered withthese three layers as well. That is, the heat-insulation layer isprovided at not only the valve head facing the combustion chamber butthe valve face positioned on its opposite side and the stem part.Accordingly, the temperature of the heat-barrier layer provided on thevalve face and the stem part can be maintained at the high temperatureby means of the heat-insulation layer, so that the heat loss at theexhaust valve can be suppressed properly. Further, the heat dissipationpath (route) made by the heat-diffusion layer can be secured so that theheat-insulation layer does not store the heat excessively, so that theexcessive temperature increase of the exhaust valve can be preventedproperly.

In the above-described combustion-chamber structure of the engine, it ispreferable that the valve be an exhaust valve with cooling function inwhich a coolant sealing portion is formed at the valve body, theheat-insulation layer and the heat-diffusion layer be provided to coverthe umbrella part of the valve, the heat-barrier layer be provided tocover the umbrella part of the valve except the contact portion of theheat-diffusion layer, and the heat-insulation layer and theheat-diffusion layer be provided to extend up to a position whichoverlaps with the coolant sealing portion of the valve body.

According to this combustion-chamber structure, the heat stored at theheat-insulation layer or the heat received by the heat-barrier layer canbe carried to the coolant sealing portion via the heat-flowing layers.Accordingly, the excessive temperature increase of the exhaust valve canbe prevented properly.

In the above-described combustion-chamber structure of the engine, it ispreferable that the heat-barrier layer be made of heat-resistant siliconresin which has the heat conductivity of 0.05-1.50 W/mK, and theheat-diffusion layer be made of copper-based material, Corson alloy,beryllium copper, fiber-reinforced aluminum alloy, or titanium aluminumwhich have the heat conductivity of 35-600 W/mK.

The present invention will become apparent from the followingdescription which refers to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view showing an engine to which acombustion-chamber structure according to an embodiment of the presentinvention is applied.

FIG. 2 is a sectional view showing details of an intake valve shown inFIG. 1.

FIG. 3 is a partially-sectional side view showing a valve of acomparative example 1.

FIG. 4 is a partially-sectional side view showing a valve of acomparative example 2.

FIG. 5 is an explanatory diagram of preignition which may be generatedin a combustion chamber of a comparative example.

FIG. 6 is a partially-sectional side view showing a valve according to afirst embodiment of the present invention.

FIG. 7 is a diagram explaining a behavior (operation) of heat in a casewhere the valve of the first embodiment is used.

FIG. 8 is a chart showing materials which are applicable to respectivestructural members of the combustion-chamber structure of the engine.

FIG. 9 is a partially-sectional side view showing an intake valveaccording to a second embodiment.

FIG. 10 is a partially-sectional side view showing an intake valveaccording to a third embodiment.

FIG. 11 is a partially-sectional side view showing an exhaust valveaccording to a fourth embodiment.

FIG. 12 is a partially-sectional side view showing an exhaust valveaccording to a fifth embodiment.

FIG. 13 is a partially-sectional side view showing an exhaust valveaccording to a sixth embodiment.

DETAILED DESCRIPTION OF THE INVENTION Entire Structure of Engine

Hereafter, a combustion-chamber structure of an engine according toembodiments of the present invention will be described specificallyreferring to the drawings. FIG. 1 is a schematic sectional view showingan engine to which the combustion-chamber structure according to theembodiments of the present invention is applied. The engine describedhere is a multi-cylinder engine which includes cylinders and pistons andis installed to the vehicle as a power source for driving the vehicle,such as an automotive vehicle. The engine includes an engine body 1,intake-exhaust manifolds, not illustrated, which are assembled to theengine body 1, and auxiliary devices, such as various kinds of pump.

The engine body 1 of the present embodiments is capable of performingthe spark-ignition combustion (SI combustion) in which the mixture offuel and air is ignited by spark in the combustion chamber and thehomogeneous-charge compression-ignition combustion (HCCI combustion) inwhich the mixture is self-ignited. A principle ingredient of the fuelsupplied to the engine body 1 is gasoline. Generally, the spark-ignitioncombustion is performed in a high-load high-speed engine operation,whereas the homogeneous-charge compression-ignition combustion isperformed in a middle/low-load middle/low-speed engine operation at theengine body 1. Herein, the present invention is applicable to acombustion chamber of the engine which is unable to perform thehomogeneous-charge compression-ignition combustion.

The engine body 1 comprises a cylinder block 3, a cylinder head 4, andpistons 5. The cylinder block 3 has plural cylinders 2 (only one ofthese is illustrated in the figure) which are arranged in a directionperpendicular to a paper plane of FIG. 1. The cylinder head 4 isattached to an upper face of the cylinder block 3 such that it closesrespective upper openings of the cylinders 2. The piston 5 is stored ineach cylinder 2 such that the piston 5 reciprocates therein, andconnected to a crankshaft 7 via a connecting rod 8. The crankshaft 7rotates around a central axis thereof according to a reciprocatingmovement of the piston 5. A cavity 5C which is concaved downwardly, in acylinder axial direction, is formed at a crown surface 5H of the piston5.

A combustion chamber 6 is partitioned above the piston 5. An intake port9 and an exhaust port 10 which respectively connect to the combustionchamber 6 are formed at the cylinder head 4. At a bottom surface 4 a(ceiling surface 6U) of the cylinder head 4 are formed an intake-sideopening portion 41 (port opening) which is a downstream end of theintake port 9 and an exhaust-side opening portion 42 (port opening)which is an upstream end of the exhaust port 10 as an opening to thecombustion chamber 6.

An intake valve 11 to open/close the intake-side opening portion 41 andan exhaust valve 12 to open/close the exhaust-side opening portion 42are assembled to the cylinder head 4. In a case of a doble overheadcamshaft (DOHC) type engine, for example, the two intake-side openingportions 41 and the two exhaust-side opening portions 42 are provided ateach of the cylinders 2, and the two intake valves 11 and the twoexhaust valves 12 are provided as well. Respective structures of theintake valve 11 and the exhaust valve 12 will be described specificallylater.

The combustion chamber 6 is partitioned by the cylinder block 3, thecylinder head 4, and the piston 5. More specifically, acombustion-chamber wall surface which partitions the combustion chamber6 comprises an inner wall surface of the cylinder 2, the piston crownsurface 5H (hereafter, referred to as “crown surface 5H” simply) whichis the upper surface of the piston 5, the combustion-chamber ceilingsurface 6U which is a bottom surface of the cylinder head 4, andrespective umbrella parts (valve heads 113, 123) of the intake valve 11and the exhaust valve 12.

An intake-side valve driving mechanism 13 and an exhaust-side valvedriving mechanism 14 which drive the intake valves 11 and the exhaustvalve 12, respectively, are provided at the cylinder head 4. Therespective stem parts of the intake valves 11 and the exhaust valve 12are driven linked with the rotation of the crankshaft 7 by these valvedriving mechanisms 13, 14. Thus, the valve head of the intake valve 11opens/closes the intake-side opening portion 41, and the valve head ofthe exhaust valve 12 opens/closes the exhaust-side opening portion 42.

The intake-side valve driving mechanism 13 comprises an intake-sidevariable valve timing mechanism (intake-side S-VT) 15. The intake-sideS-VT 15 is an electrical type of S-VT which is provided at an intakecamshaft and configured to change an opening/closing timing of theintake valve 11 by continuously changing a rotational phase of theintake camshaft relative to the crankshaft 7 within a specified anglerange. Likewise, the exhaust-side valve driving mechanism 14 comprisesan exhaust-side variable valve timing mechanism (exhaust-side S-VT) 16.The exhaust-side S-VT 16 is an electrical type of S-VT which is providedat an exhaust camshaft and configured to change an opening/closingtiming of the exhaust valve 12 by continuously changing a rotationalphase of the exhaust camshaft relative to the crankshaft 7 within aspecified angle range.

A single spark plug 17 to supply ignition energy to the mixture in thecombustion chamber 6 is attached to the cylinder head 4 for eachcylinder 2. The spark plug 17 is attached to the cylinder head 4 suchthat it is arranged at a central space, in a radial direction, ofcombustion chamber 6 and its ignition point is exposed to an insidespace of the combustion chamber 6. The spark plug 17 discharges a sparkfrom its tip according to a power supply from an ignition circuit, notillustrated, thereby igniting the mixture in the combustion chamber 6.The ignition plug 17 of the present embodiments is used to perform thespark-ignition combustion in the high-load high-speed engine operation.Further, this is also used, when the homogeneous-chargecompression-ignition combustion is performed, in a case where it is hardto perform the self-ignition right after an engine start during a coldtime, the homogeneous-charge compression-ignition combustion is assistedunder a specified load or speed conditions (spark assist), or the like.

A single injector 18 to inject the gasoline, as the principle ingredientof the fuel, from its tip portion into the combustion chamber 6 isattached to the cylinder head 4 for each cylinder 2. A fuel supply pipe19 is coupled to the injector 18. The injector 18 injects the fuelsupplied through the fuel supply pipe 19 toward the cavity 5C. Ahigh-pressure fuel pump (not illustrated) which includes a plunger typeof pump and the like and is operationally connected to the crankshaft 7is coupled to an upstream side of the fuel supply pipe 19. A common railfor pressure accumulation which is common to the all cylinders 2 isprovided between the high-pressure fuel pump and the fuel supply pipe19. The fuel pressure-accumulated in the common rail is supplied to theinjector 18 of each cylinder 2, and the high-pressure fuel is injectedfrom the injector 18 into the combustion chamber 6.

Specific Structure of Valve

Subsequently, a specific structure of the intake valve 11 (valve) willbe described. Herein, a basic structure of the exhaust valve 12 issimilar to the intake valve 11. FIG. 2 is a sectional view showingdetails of the intake valve 11. The intake valve 11 (exhaust valve 12)is a so-called poppet valve and comprises an intake valve body 110(valve body) which comprises an umbrella part 111 and a stem part 112.

The umbrella part 111 comprises a valve head 113 which faces thecombustion chamber 6 and a valve face 114 which is positioned on anopposite side to the combustion chamber 6. As described above, the valvehead 113 is a combustion-chamber wall surface which partitions a part ofthe combustion chamber 6. The stem part 112 comprises a tip section 112Awhich is connected to the umbrella part 111 and a base end section 112Bto which a driving force is applied from the intake-side valve drivingmechanism 13. The stem part 112 is held by a valve guide 131 so as tomove in an axial direction.

A valve spring 132 is attached around the stem part 112. The valvespring 132 is interposed between the a spring seat 133 which is fixedaround the base end section 112B and an upper face of the cylinder head4. The valve spring 132 presses the spring seat 133, so that the intakevalve 11 is biased in a direction in which the umbrella part 111 sealsthe intake-side opening portion 41 (in an upward direction).

A ring-shaped valve seat 4S is provided at an opening edge of theintake-side opening portion 41. A part of the umbrella part 111 contactswith the valve seat 4S. Specifically, a portion of the valve face 114around its outer peripheral edge contacts with an inner peripheral wallof the valve seat 4S when the intake valve 11 is closed. This contactingof the umbrella part 111 with the valve seat 4S makes the intake port 9and the combustion chamber 6 be shut off, so that the combustion chamber6 is sealed. Meanwhile, when the intake valve 11 is opened, the umbrellapart 111 moves separately from the valve seat 4S according to a downwardmove of the intake valve 11, so that the intake port 9 and thecombustion chamber 6 have a connection state.

Explanation of Comparative Examples of Valve

Before describing a valve according to the embodiments of the presentinvention, valves of the comparative examples will be described. FIG. 3is a partially-sectional side view showing an intake valve 11P1 (anexhaust valve is similar to this intake valve) of a comparativeexample 1. The intake valve 11P1 has a heat-barrier layer 720 only.Specifically, the intake valve 11P1 comprises the intake valve body 110including the umbrella part 111 and the stem part 112, and an entireportion of the umbrella part 111 (the valve head 113 and the valve face114) is covered with the heat-barrier layer 720. The heat-barrier layer720 is a coating layer which is made of a material having thesufficiently-smaller heat conductivity than the intake valve body 110,such as heat-resistant silicon resin.

The umbrella part 111 of the intake valve 11P1, especially the valvehead 113, faces the combustion chamber 6, so that it is exposed to thehigh temperature. In a case of an intake/exhaust four-valve type ofengine body 1, for example, an area which the four valve heads 113occupy shows a considerably large rate relative to an entire area of thecombustion-chamber wall surface. Accordingly, it is necessary to takesome countermeasures for suppressing heat loss through the intake valve11P1.

The heat-barrier layer 720 covering the umbrella part 111 is the layerhaving the small heat conductivity, and therefore its temperaturechanges depending on the temperature of an inside of the combustionchamber 6. Therefore, a difference between the temperature of thecombustion gas inside the combustion chamber 6 and the surfacetemperature of the umbrella part 111 is made so small that heat transferto the intake valve body 110 can be blocked to a certain degree.Accordingly, the heat loss can be reduced to a certain degree. However,the heat-barrier layer 720 is generally a thin layer which is made of amaterial having the small volume specific heat. Therefore, theheat-barrier layer 720 has the poor (inferior) heat-storage performanceand cannot block the heat transfer to the intake valve body 110perfectly, so that the heat loss cannot be reduced sufficiently.

FIG. 4 is a partially-sectional side view showing an intake valve 11P2(an exhaust valve is similar to this intake valve) of a comparativeexample 2. The intake valve 11P2 has a heat-insulation layer 710 inaddition to the heat-barrier layer 720. The intake valve 11P2 of thecomparative example 2 is the same as the intake valve 11P1 of thecomparative example 1 regarding a structure in which an entire portionof the umbrella part 111 of the intake valve 11P2 is covered with theheat-barrier layer 720. Further, in an area of the intake valve 11P2which corresponds to the valve head 113, the heat-insulation layer 710is arranged adjacently to a back-face side of the heat-barrier layer720. That is, the valve head 113 is covered with two layers of theheat-insulation layer 710 and the heat-barrier layer 720 positionedabove the heat-insulation layer 710.

The heat-insulation layer 710 is made of a material having the largevolume specific heat and has the heat-storage performance. Theheat-insulation layer 710 stores the heat which has passed through theheat-barrier layer 720. Therefore, the heat-insulation layer 710 heats(retains the heat of) the heat-insulation layer 710 provided on thevalve head 113. Accordingly, the surface temperature of the valve head113 is made high, so that a difference between the surface temperatureof the valve head 113 and the temperature of the combustion gas in thecombustion chamber 6 can be made small. In other words, the heattransfer from the combustion chamber 6 to the intake valve body 110 isblocked, so that the heat dissipation is suppressed. Consequently, theheat loss can be reduced considerably.

Herein, according to the research conducted by the inventors and others,it has been found that the structure of the intake valve 11P2 has thefollowing problems. In a case where the temperature inside thecombustion chamber 6 is not made relatively high, for example, when thehomogenous-charge compression-ignition combustion using the lean mixtureis performed in the low-load engine operation, the intake valve 11P2 ofthe comparative example 2 works effectively. That is, theheat-insulation layer 710 retains an appropriate stored temperature,thereby heating the heat-barrier layer 720 properly. Accordingly, thesurface of the valve head 113 can be made to reach the temperature whichis suitable for suppressing the heat loss.

Meanwhile, in a case where the temperature inside the combustion chamber6 is made relatively high, the heat-insulation layer 710 stored the hightemperature heats the heat-barrier layer 720 excessively. The enginebody 1 performs the homogenous-charge compression-ignition combustionusing the lean mixture in the middle-load engine operation and performsthe spark-ignition combustion with the air-fuel ratio: 2=1 in thehigh-load engine operation, for example. Since the amount of fuelinjection becomes relatively large in the middle/high-load engineoperation, the temperature of the combustion gas in the combustionchamber 6 becomes relatively high. Therefore, the valve head 113 comesto receive the high temperature as well, so that the heat-insulationlayer 710 comes to store the high heat as well. Since the heat-barrierlayer 720 is heated by this heat-insulation layer 710, the surfacetemperature of the valve head 113 becomes considerably high.

FIG. 5 is a diagram showing a phenomenon which may occur in themiddle/high-load engine operation in a combustion-chamber structureusing the intake valve 11P2 of the comparative example. In FIG. 5, notonly the intake valve 11P2 but the exhaust valve 12P2 having the similarstructure thereto are shown. The exhaust valve 12P2 comprises anumbrella part 121 and a stem part 122, and the umbrella part 121comprises the heat-insulation layer 710 and the heat-barrier layer 720which are similar to those of the intake valve 11P2.

When the heat-insulation layer 710 stores the high-temperature heat andthe heat-barrier layer 720 is heated by this heat, the respective valveheads 113, 123 of the intake valve 11P1 and the exhaust valve 12P2 cometo have the high temperature. The valve heads 113, 123 which have beenexcessively heated generate the heat operative to heat the combustionchamber 6 (an arrow H in FIG. 5), so that the cylinder internaltemperature is made excessively high. Accordingly, the temperature ofthe air taken into the combustion chamber 6 in an intake stroke of theengine increases, and when this air having the increased is compressedin a compression stroke of the engine, preignitions PIG may occur. Thatis, there may occur the phenomenon in which a part of the mixture isignited at an earlier timing than a normal (appropriate)compression-ignition timing. In this case, some problems, such as animproper torque fluctuation or output decrease of the engine body 1, maybe caused.

Description of Embodiments of Valve

The present embodiments provide combustion-chamber structures which cansuppress of occurrence of the preignitions PIG shown in FIG. 5, reducingthe heat loss through the intake valve 11 and the exhaust valve 12. Inthe embodiments 1-6 described below, various structures of the intakevalve 11 and the exhaust valve 12 which can provide the above-describedcombustion-chamber structure will be exemplified.

Embodiment 1

FIG. 6 is a partially-sectional side view showing the intake valve 11according to a first embodiment. FIG. 7 is an enlarged view of asectional portion of the intake valve 11 of FIG. 6, which shows apositional relationship of the intake valve 11 with the vale seat 4S(cylinder head 4). The structure of the intake valve 11 which is shownhere is applicable to the exhaust valve 12. The intake valve 11comprises the intake valve body 110 which includes the umbrella part 111and the stem part 112, the heat-insulation layer 71 and the heat-barrierlayer 72 which are shown in the comparative example 2 as well, and theheat-diffusion layer 73 which is not provided in the comparative example2.

The heat-insulation layer 71 is provided at the valve seat 113 of theumbrella part 111. The heat-insulation layer 71 has a specifiedthickness in a valve shaft (axial) direction and is of a circular shapewhich is similar to the valve head 113 in a plan view of the valve shaftdirection. A circular-shaped outer peripheral edge 711 of theheat-insulation layer 71 extends up to a position near an outerperipheral edge of the valve head 113 (umbrella part 111). Of course,this circular shape, in the plan view, of the heat-insulation layer 71is just one example, and this layer 71 may have any other shape, such asa polygon. Further, the heat-insulation layer 71 may have a smaller sizethan the valve head 113, and the heat-insulation layer 71 may beprovided only in a central area, in a radial direction, of the valvehead 113, for example. The thickness, in the valve shaft direction, ofthe heat-insulation layer 71 can be selected from a range of 1-6 mm, forexample.

It is preferable that the heat conductivity of the heat-insulation layer71 be as small as possible from viewpoints of suppressing the heat fromescaping from the combustion chamber 6 through the intake valve 11 (theexhaust valve 12) (suppression of the heat loss), and at least amaterial which has the smaller heat conductivity than the intake valvebody 110 (an exhaust valve body 120) is used. Further, it is preferablethat the heat-insulation layer 71 have the volume specific heat which isas large as possible, i.e., the high heat-storage performance, fromviewpoints of maintaining the valve head 113 at the high temperature.

The heat-barrier layer 72 is provided to cover the valve head 113 wherethe heat-insulation layer 71 is provided for suppression of the heatloss through the intake valve body 110. That is, the heat-insulationlayer 72 is exposed to the surface of the valve head 113. The heatconductivity of the heat-barrier layer 72 is set to be smaller thanthose of the intake valve body 110 and the heat-insulation layer 71 fromviewpoints of suppressing the heat from escaping from the valve head 113to the intake valve body 110. By providing the heat-barrier layer 72,the temperature difference between the combustion gas generated in thecombustion chamber 6 and the valve head 113 can be made small, therebyreducing the heat loss. The thickness, in the valve shaft direction, ofthe heat-barrier layer 72 can be selected from a range of 0.03-0.25 mm,for example.

The heat-diffusion layer 73 is provided between the heat-insulationlayer 71 and the heat-barrier layer 72 such that itscombustion-chamber-side face contacts with the heat-barrier layer 72 andits opposite-side face contacts with the heat-barrier layer 72. Theheat-diffusion layer 73 is the layer which has the function of makingthe heat stored at the heat-insulation layer 71 escape to the cylinderhead 4 so that the temperature of the valve head 113 where theheat-insulation layer 71 is provided does not become too high. It ispreferable that the heat conductivity of the heat-diffusion layer 73 beas large as possible from viewpoints of immediate transfer of the heatstored at the heat-insulation layer 71 to the cylinder head 4.Therefore, the heat-diffusion layer 73 is configured to have the largerheat conductivity than the heat-insulation layer 71 and the heat-barrierlayer 72. The thickness, in the valve shaft direction, of theheat-diffusion layer 73 can be selected from a range of 1-5 mm, forexample. Herein, it is preferable that the heat resistance, which isrepresented by “heat conductivity/thickness,” of the heat-diffusionlayer 73 be as small as possible from viewpoints of appropriate heatdiffusion. Therefore, the thickness of the heat-diffusion layer 73 isset properly considering the heat conductivity of the material of whichthe heat-diffusion layer 73 is made.

Referring to FIG. 7, the heat-diffusion layer 73 comprises a contactportion 731 which is provided to extend up to a position of a part ofthe valve face 114 from the outer peripheral edge of the valve head 113.A portion around an outer peripheral edge (a portion having the largestdiameter) of the valve face 114 becomes a contact face CP (contactingposition) of the umbrella part 111 which contacts with the valve seat 4Swhen the intake valve 11 is closed. The above-described contact portion731 extends up to the position of the contact face CP. That is, thecontact portion 731 is located at a position which directly contactswith a reception face 43 of the valve seat 4S when the intake valve 11is closed. The heat-diffusion layer 73 receives the heat which isexcessively stored at the heat-insulation layer 71 and makes this heatescape from the contact portion 731 to the cylinder head 4 through thevalve seat 4S.

An operation (move) of the above-described heat dissipation (heatescaping) will be described referring to arrows D1-D3 show in FIG. 7. Asshown by the arrow D1, since the heat-barrier layer 72 has theextremely-low heat conductivity and changes its temperature depending onthe chamber temperature of the combustion chamber 6, the heat transferfrom the combustion gas in the combustion chamber 6 to the intake valvebody 110 can be blocked considerably. That is, the heat can be preventedfrom escaping from the combustion chamber 6 through the valve head 113.Thereby, the heat loss can be reduced. However, since the heat-barrierlayer 72 cannot block the heat transfer perfectly, the heat is made topass through to a certain degree as shown by the arrow D2. Theheat-insulation layer 71 of the present embodiment is made of thematerial having the large volume specific heat, thereby providing thesuperior heat-storage performance. Accordingly, the heat passed throughthe heat-barrier layer 72 (the arrow D2) and the surrounding heat arestored at the heat-insulation layer 71.

Then, the heat-insulation layer 71 which has stored the heat comes toheat the heat-barrier layer 72. Accordingly, the valve head 113 wherethe heat-insulation layer 71 is provided can be maintained at the hightemperature. However, as described regarding the comparative example 2,the heat-insulation layer 71 stores the high-temperature heat in acertain engine operation where the temperature of the combustion gas isrelatively high. Accordingly, the heat-insulation layer 72 isexcessively heated, so that the preignition is caused. In order toprevent this problem, the heat-diffusion layer 73 is provided betweenthe heat-insulation layer 71 and the heat-barrier layer 72 such that theheat-diffusion layer 73 receives the heat stored at the heat-insulationlayer 71. Further, as shown by the arrow D3, when the contact portion731 contacts with the valve seat 4S, the heat-diffusion layer 73 makesthe heat received from the heat-insulation layer 71 escape to the valveseat 4S. This heat is transferred from the valve seat 4S to the cylinderhead 4. Accordingly, the excessively high temperature of the valve head113 is so suppressed that the preignition can be prevented fromoccurring previously.

Subsequently, examples of the material which can be appropriately usedas a structural member of the combustion chamber 6 are shown. A castingof a metal-based material, such as aluminum alloy AC4B (the heatconductivity=96 W/mK, the volume specific heat=2667 kJ/m³K), can be usedas respective base materials of the cylinder block 3 and the cylinderhead 4. Further, aluminum alloy AC8A (the heat conductivity=125 W/mK,the volume specific heat=2600 kJ/m³K) can be used as a base material ofthe piston 5 (piston body 50).

Heat-resistant steel which is superior in the heat-resistantperformance, the wear-resistant performance, and the corrosion-resistantperformance can be used for the intake valve body 110 and the exhaustvalve body 120. Martensite-based heat-resistant steel SUH11 based onchrome, silicon, and carbon (the heat conductivity=25 W/mK, the volumespecific heat=3850 kJ/m³K) can be used for the intake valve body 110,for example. Martensite-based heat-resistant steel SUH35 based onchrome, silicon, and carbon (the heat conductivity=18 W/mK, the volumespecific heat=3565 kJ/m³K) can be used for the exhaust valve body 120,for example.

Like the above-described examples, it is preferable that the cylinderhead 4 have the larger heat conductivity than the intake valve body 110and the exhaust valve body 120. Since the contact portion 731 of theheat-diffusion layer 73 contacts with the valve face 114 of the umbrellapart 111, the heat can be made to escape to the intake valve body 110 aswell. However, by setting the heat conductivity of the cylinder head 4to be larger than those of the intake valve body 110 and the exhaustvalve body 120, the heat can be made to escape from the contact portion731 to the valve bodies 110, 120 actively.

The material which has the smallest heat conductivity and the smallestvolume specific heat is selected for the heat-barrier layer 72 among thestructural members of the intake valve 11 and the exhaust valve 12 (theintake valve body 110 and the exhaust valve body 120, theheat-insulation layer 71, the heat-barrier layer 72, and theheat-diffusion layer 73). That is, the appropriate material of theheat-barrier layer 72 is selected such that this layer 72 does notdiffuse the heat easily and does not store the heat easily. A range ofthe preferable heat conductivity of the heat-barrier layer 72 is about0.05-1.50 W/mK, and a range of the preferable volume specific heat ofthe heat-barrier layer 72 is about 500-1500 kJ/m³K.

For example, the heat-resistant silicon resin can be exemplified as thematerial of the heat-barrier layer 72 which meets the above-describedrequirements. The silicon resin made of three-dimensional polymer havingthe high branching degree which is represented by methyl silicon resinand methylphenyl silicon resin can be exemplified as the above-describedsilicon resin, and polyalkylphenylsiloxane or the like are preferablyused, for example. This silicon resin may contain microballoonparticles, such as Shirasu balloons. The heat-barrier layer 72 can beformed by a coating process in which the above-described silicon resinis coated on the valve face 114 of the umbrella part 111 where theheat-insulation layer 71 and the heat-diffusion layer 73 are formed, forexample.

The heat-insulation layer 71 does not diffuse the heat easily but storesthe heat easily. The material which has the larger heat conductivitythan the heat-barrier layer 72 and the extremely-smaller heatconductivity than the intake valve body 110 and the exhaust valve body120 is selected for the heat-insulation layer 71 in order to suppressthe heat diffusion. Further, the material which has the larger volumespecific heat and the larger heat resistance than the heat-barrier layer72 is selected for the heat-insulation layer 71 in order to provide theappropriate heat-storage performance A range of the preferable heatconductivity of the heat-insulation layer 71 is about 0.2-10 W/mK, and arange of the preferable volume specific heat of the heat-insulationlayer 71 is about 1800-3500 kJ/m³K.

A ceramics material can be exemplified as the material of theheat-insulation layer 71 which meets the above-described requirements,for example. In general, since the ceramics material has the small heatconductivity but has the large volume specific heat and the superiorheat resistance, this material is suitable for the heat-insulation layer71. Specifically, a preferable ceramics material is zirconia (the heatconductivity=3 W/mK, the volume specific heat=2576 kJ/m³K).Alternatively, the ceramics material, such as silicon nitride, silica,cordierite, or mullite, a porous SUS based material, calcium silicate,or the like can be used as the material of the heat-insulation layer 71as well.

The heat-diffusion layer 73 makes the heat stored at the heat-insulationlayer 71 escape to the cylinder head 4 from the contact portion 731, andtherefore this layer 73 is the layer which easily diffuses the heat.Thus, the heat-diffusion layer 73 has the largest heat conductivityamong the structural members of the intake valve 11 or the exhaust valve12. A range of the preferable heat conductivity of the heat-diffusionlayer 73 is about 35-600 W/mK. Further, it is preferable that thethickness of the heat-diffusion layer 73 be set such that the heatresistance is within a range of 0.002-0.06 m²K/W. A copper-basedmaterial (the heat conductivity=400 W/mK, the volume specific heat=3500kJ/m³K), Corson alloy, beryllium copper, fiber-reinforced aluminumalloy, titanium aluminum, or the like can be used as the material of theheat-diffusion layer 73 which meets the above-described requirements.The above-described copper-based material is particularly preferablebecause even in a case where the thickness is set at 2 mm, the heatresistance of the heat-diffusion layer 73 can be controlled (suppressed)at a value of 0.005 m²K/W.

FIG. 8 shows a preferred material selection example of the basematerials of the intake valve body 110 and the exhaust valve body 120(valve base materials), the cylinder block 3, the cylinder head 4 andthe piston 5, the heat-insulation layer 71, the heat-barrier layer 72,and the heat-diffusion layer 73. FIG. 8 shows the heat conductivity λ,the volume specific heat ρc, the heat diffusivity (λ/ρc), theZ-directional thickness t, the heat resistance (t/λ), and the heatpermeability (√λρc) of each of these materials. Herein, a right-sidesmall column of the heat diffusivity shows each value of the respectivelayers in a case where the heat diffusivity of the heat-barrier layer 72is considered as “1”.

Embodiment 2

In a second embodiment, a preferable example as the intake valve will beshown. FIG. 9 is a partially-sectional side view showing an intake valve11A according to the second embodiment. The intake valve 11A comprisesthe intake valve body 110 including the umbrella part 111 and the stempart 112, the heat-insulation layer 71, the heat-barrier layer 72, andthe heat-diffusion layer 73. What is different from the above-describedfirst embodiment is that there is provided a valve-face heat-barrierlayer 721 which covers over the valve face 114 of the umbrella part 111.

The valve-face heat-barrier layer 721 covers the valve face 14 over anarea from a base end section 111A (a portion connected to the tipsection 112A of the stem part 112) of the umbrella part 111 to aposition of the umbrella part 111 which partially overlaps with thecontact portion 731. That is, the structure in which the contact portion731 is exposed to the contact face CP of the umbrella part 111 is thesame as that of the first embodiment. The valve-face heat-insulationlayer 721 can have the same material and thickness as the heat-barrierlayer 72. Herein, the heat-barrier layer 721 may be provided to extendup to a position located above a portion of the stem part 112 (aspecified length from the tip section 112A toward the base end section112B).

Even if the heat-insulation layer 71 and the heat-barrier layer 72 arearranged on the side of the valve head 113, the intake valve body 110has the heat. The higher the temperature of the combustion chamber inthe combustion chamber becomes, the higher the temperature of the intakevalve body 110 becomes. As shown in FIG. 2, the umbrella part 111 of theintake valve body 110 is positioned in the intake port 9 when the valve11 is closed. At this moment, the intake air inside the intake port 9contacts with the valve face 114 of the umbrella part 111. Further, whenthe valve 11 is opened as well, the intake air flowing from the intakeport 9 into the combustion chamber 6 hits against the valve face 114.Therefore, if the temperature of the surface of the valve face 114becomes high, the temperature of the intake air is increased. If theexcessively-heated intake air is introduced into the combustion chamber6 in the intake stroke of the engine, the preignition occurs in thecompression stroke of the engine.

The valve-face heat-barrier layer 721 prevents the heat of the umbrellapart 111 from escaping to the outside from the valve face 114. That is,as shown by the arrow D4 in FIG. 9, the valve-face heat-barrier layer721 performs the function of capturing the heat inside the umbrella part111. Accordingly, even in a case where the temperature of the umbrellapart 111 of the intake valve 11 becomes high, the heat can be preventedfrom escaping from the valve face 114. Accordingly, the temperature ofthe intake air passing through the intake port 9 is suppressed fromincreasing excessively, so that the preignition can be prevented.

Herein, the heat of the umbrella part 111 is made to escape to thecylinder head through the contact portion 731 of the heat-diffusionlayer 73 to a certain degree. This point is similar to a case of theintake valve 11 of the first embodiment in which the valve-faceheat-barrier layer 721 is not provided. However, since there are manycases where the above-described heat dissipation (escaping) through thecontact portion 731 is not enough, it is preferable that the heatdissipation from the valve face 114 be suppressed by providing thevalve-face heat-barrier layer 721 like the present embodiment.

Embodiment 3

A preferable example of the intake valve will be shown as a thirdembodiment as well. FIG. 10 is a partially-sectional side view showingan intake valve 11B according to the third embodiment. The intake valve11B comprises the intake valve body 110 including the umbrella part 111and the stem part 112, the heat-insulation layer 71, the heat-barrierlayer 72, and the heat-diffusion layer 73. What is different from theabove-described second embodiment is that there is provided a stem-partheat-barrier layer 722 which is provided to cover over the stem part 112in addition to the valve-face heat-barrier layer 721, and theheat-diffusion layer 73 extends from the valve face 114 to the stem part112.

Similarly to the second embodiment, the valve-face heat-barrier layer721 is provided on the valve face 114 in an area from an end edge of thecontact portion 731 to the base end section 111A. The stem-partheat-barrier layer 722 is provided on the stem part 112 such that it iscontinuous to the valve-face heat-barrier layer 721. That is, thevalve-face heat-barrier layer 721 and the stem-part heat-barrier layer722 cover the valve face 114 and the stem part 112 except the contactportion 731. It is preferable that the stem-part heat-barrier layer 722cover around the tip section 112A of the stem part 112, especially asection of the stem part 112 which projects downwardly from the valveguide 131 (FIG. 2).

The heat-diffusion layer 73 comprises a valve-head heat-diffusion layer730 (first portion), the above-described contact portion 731, avalve-face heat-diffusion layer 732 (second portion), and a stem-partheat-diffusion layer 733 (second portion). The valve-head heat-diffusionlayer 730 is provided between the heat-insulation layer 71 and theheat-barrier layer 72 at the valve head 113. The contact portion 731 isexposed to the contact face CP of the stem part 111 and contacts withthe valve seat 4S similarly to the first and second embodiments. Thevalve-face heat-diffusion layer 732 is a base layer of the valve-faceheat-barrier layer 721 which is provided on the valve face 114. Thestem-part heat-diffusion layer 733 is a base layer of the stem-partheat-barrier layer 722 which is provided on the stem part 112.

The intake valve 11 receives the heat from the exhaust gas in somecases. For example, in a case where a closing timing of the intake valveis delayed by setting a valve overlap term, there may occur blow back ofthe exhaust gas from the combustion chamber 6 to the intake port 9 afterthe combustion. In this case, the intake valve 11 is heated by thisblow-back exhaust gas. Especially, the umbrella part 111 is heated.Further, the intake valve 11 may be heated by the EGR gas as well. Thus,the intake valve 11 heated by the exhaust gas may increase thetemperature of the intake air excessively.

Herein, the intake valve 11B of the third embodiment is configured suchthat the heat-diffusion layer 73 is provided with the valve-faceheat-diffusion layer 732 and the stem-part heat-diffusion layer 733 andthereby the heat received by the umbrella part 111 and the stem part 112escapes from the contact portion 731 to the cylinder head 4. That is, asshown by the arrow D4 in FIG. 10, the valve-face heat-barrier layer 721performs the function of capturing the heat inside the umbrella part111. This is the same as the second embodiment. Further, as shown by anarrow D41, the stem-part heat-barrier layer 722 performs the function ofcapturing the heat inside the stem part 112. Therefore, even in a casewhere the temperature of the umbrella part 111 and the stem part 112 ofthe intake valve 11 is made high, the heat dissipation from the surfacesof the valve face 114 and the stem part 112 can be suppressed.

Meanwhile, in a case where the valve-face heat-barrier layer 721 and thestem-part heat-barrier layer 722 are exposed to the high temperaturethrough contacting with the exhaust gas and the like that, theheat-diffusion layer 73 performs of the function of the heatdissipation. That is, the valve-face heat-diffusion layer 732 receivesthe heat from the valve-face heat-barrier layer 721, and the stem-partheat-diffusion layer 733 receives the heat from the stem-partheat-barrier layer 722. This heat is made to escape from the contactportion 731 to the cylinder head 4 as shown by the arrow D5.Accordingly, the excessively-high temperature of the intake valve 11Bcan be prevented.

Embodiment 4

A preferable example of the exhaust valve will be shown as a fourthembodiment as well. FIG. 11 is a partially-sectional side view showingan exhaust valve 12A according to the fourth embodiment. The exhaustvalve 12A comprises the intake valve body 120 including the umbrellapart 121 and the stem part 122, the heat-insulation layer 71, theheat-barrier layer 72, the heat-diffusion layer 73, and a valve-faceheat-barrier layer 721 which is provided to cover over a valve face 124of the umbrella part 121. A layer structure of the exhaust valve 12 ofthe fourth embodiment is the same as that of the intake valve 11A of thesecond embodiment.

The valve-face heat-barrier layer 721 covers the valve face 124 in anarea from a base end section 121A to a position which partially overlapswith the contact portion 731 of the heat-diffusion layer 73. Similarlyto the above-described embodiments, the contact portion 731 is exposedto the contact face CP of the umbrella part 121 with the valve seat 4S.Herein, the heat-barrier layer 721 may be provided to extend up to aposition located above a portion of the stem part 122 (a specifiedlength from the tip section 112A toward the base end section 112B).

The exhaust valve 12A which is provided at the exhaust port 10 isexposed to the high temperature by the exhaust heat of the combustiongas. Differently from the second embodiment, the valve-face heat-barrierlayer 721 suppresses the umbrella part 121 from receiving the exhaustheat. The temperature of the valve-face heat-barrier layer 721 increasesthrough contacting with the high-temperature exhaust gas, so that thetemperature difference between the exhaust gas and the valve face 124 ismade small. Accordingly, as shown by the arrow D6 in FIG. 11, the heattransfer of the exhaust heat to the umbrella part 121 through the valveface 124 can be suppressed by providing the valve-face heat-barrierlayer 721. That is, the heat loss can be suppressed.

Embodiment 5

A preferable example of the exhaust valve will be shown as a fifthembodiment as well. FIG. 12 is a partially-sectional side view showingan exhaust valve 12B according to the fifth embodiment. The exhaustvalve 12B comprises the structure in which the exhaust valve body 120 iscovered with three layers of the heat-insulation layer, the heat-barrierlayer, and the heat-diffusion layer except the contact portion 731.While the fifth embodiment shows an example in which a section of thestem part 122 which is continuous to the umbrella part 121 is coveredwith the above-described three layers, only the umbrella part 121 may becovered with the above-described three layers.

The exhaust valve 12B comprises the heat-insulation layer 71 which isprovided to correspond to the valve head 123, a valve-faceheat-insulation layer 712 which is provided to extend from the outerperipheral edge 711 of the heat-insulation layer 71 onto the valve face124, and a stem-part heat-insulation layer 713 which is provided on thestem part 122 as the heat-insulation layers. The exhaust valve 12Bfurther comprises the valve-face heat-barrier layer 721 and thestem-part heat-barrier layer 722 covering the stem part 122 as theheat-barrier layers, in addition to the heat-barrier layer 72 coveringthe valve head 123.

The heat-diffusion layer 73 comprises the valve-head heat-diffusionlayer 730 (first portion), the above-described contact portion 731, thevalve-face heat-diffusion layer 732 (second portion), and the stem-partheat-diffusion layer 733 (second portion). The valve-head heat-diffusionlayer 730 is provided between the heat-insulation layer 71 and theheat-barrier layer 72 at the valve head 123. The contact portion 731 isexposed to the contact face CP of the umbrella part 121 and contactswith the valve seat 4S. The valve-face heat-diffusion layer 732 isprovided between the valve-face heat-insulation layer 712 and thevalve-face heat-barrier layer 721 at the valve face 124. The stem-partheat-diffusion layer 733 is provided between the stem-partheat-insulation layer 713 and the stem-part heat-barrier layer 722 atthe stem part 122.

The valve-face heat-barrier layer 721 suppresses the umbrella part 121from receiving the exhaust heat similarly to the fourth embodiment. Asshown by the arrow D6 in shown in FIG. 12, it can be suppressed byproviding the valve-face heat-barrier layer 721 that the exhaust heat istransferred to the umbrella part 121 through the valve face 124. Thestem-part heat-barrier layer 722 suppresses the exhaust heat from beingtransferred to the stem part 122 as shown by an arrow D61 similarly tothe stem-part heat-barrier layer 722. The valve-face heat-insulationlayer 712 and the stem-part heat-insulation layer 713 are provided tomaintain the respective temperatures of the valve-face heat-insulationlayer 721 and the stem-part heat-barrier layer 722 at a hightemperature. The valve-face heat-insulation layer 712 and the stem-partheat-insulation layer 713 store the heat which passes through thevalve-face heat-insulation layer 721 and the stem-part heat-barrierlayer 722 as shown by the arrow D7, and heat the heat-barrier layers721, 722. Thereby, the heat loss through the exhaust valve 12B can besecurely suppressed.

The valve-face heat-diffusion layer 732 and the stem-part heat-diffusionlayer 733 are provided to make the heat stored at the valve-faceheat-insulation layer 712 and the stem-part heat-insulation layer 713escape. The valve-face heat-diffusion layer 732 receives the heat fromthe valve-face heat-insulation layer 712 and the valve-faceheat-insulation layer 721. The stem-part heat-diffusion layer 733receives the heat from the stem-part heat-insulation layer 713 and thestem-part heat-barrier layer 722. This heat is made to escape from thecontact portion 731 to the cylinder head 4 as shown by the arrow D8.That is, even in a case where the valve-face heat-insulation layer 712and the stem-part heat-insulation layer 713 store the heat at theexcessively high temperature, a heat dissipation route formed by theheat-diffusion layers 732, 733 is secured. Herein, the valve-headheat-diffusion layer 730 performs the heat-dissipation function of theheat-insulation layer 71 of the valve head 123, which is the same as theother embodiments. Accordingly, the excessive increase of thetemperature of the exhaust valve 12B which may cause the preignition canbe prevented properly.

Embodiment 6

A preferable example of the exhaust valve will be shown as a sixthembodiment as well. FIG. 13 is a partially-sectional side view showingan exhaust valve 12C according to the sixth embodiment. The exhaustvalve 12C is the valve having the cooling function in which a coolantsealing portion 125 is formed at the exhaust valve body 120. The sealingportion 125 extends from the stem part 122 up to an area of the umbrellapart 121 which is positioned on a slightly-deep side of the base endsection 121A. A coolant which is sealed into the coolant sealing portion125 is metallic sodium, for example.

The heat-insulation layer 71 and the heat-diffusion layer 73 of theexhaust valve 12C are provided to cover the umbrella part 121. Theheat-barrier layer 72 is provided to cover the umbrella part 121 exceptthe contact portion 731. Further, the heat-barrier layer 72 and theheat-diffusion layer 73 extend up to the stem part 122 such that theyoverlap with the coolant sealing portion 125. That is, similarly to theabove-described embodiment 5, the three-layer structure of theheat-insulation layer 71, the heat-barrier layer 72, and theheat-diffusion layer 73 is formed in the area from the umbrella part 121to an end edge of the coolant sealing portion 125 except the contactportion 731. Meanwhile, a two-layer structure of the heat-barrier layer72 and the heat-diffusion layer 73 is formed in an area which overlapswith the coolant sealing portion 125.

Specifically, the valve-face heat-diffusion layer 732 is providedbetween the valve-face heat-insulation layer 712 and the valve-faceheat-barrier layer 721 at the valve face 124. Meanwhile, the stem-partheat-diffusion layer 733 contacts with the stem-part heat-barrier layer722 at its outside face, but its inside face contacts with a surface ofthe stem part 122. That is, the stem-part heat-diffusion layer 733 facesthe coolant sealing portion 125.

The heat transfer to the exhaust valve body 120 can be suppressed byproviding the valve-face heat-barrier layer 721 and the stem-partheat-barrier layer 722 as shown by the arrows D6, D61 in FIG. 13. Thevalve-face heat-insulation layer 712 stores the heat which has passedthrough the valve-face heat-barrier layer 721 (arrow D7) and maintainsthe heat-barrier layer 721 at the high temperature. In a case where thevalve-face heat-insulation layer 712 is heated at the high temperatureexcessively, as shown by the arrow D8, this high-temperature heat istransferred to the valve-face heat-diffusion layer 732 and made toescape from the contact portion 731 to the cylinder head 4.

The heat of the heat-insulation layer 71 and the valve-faceheat-insulation layer 712 is made to escape to the coolant sealingportion 125 as well by the heat-diffusion layer 73. That is, theheat-diffusion layer 73 is configured such that the valve-headheat-diffusion layer 730 which contacts with the heat-insulation layer71 and the valve-face heat-diffusion layer 732 which contacts with thevalve-face heat-insulation layer 712 are connected by the contactportion 731, and the heat-diffusion layer 73 comprises the stem-partheat-diffusion layer 733 which faces the coolant sealing portion 125.Therefore, the heat of the heat-insulation layers 71, 712 is transferredto the stem-part heat-diffusion layer 733 and made to escape to thecoolant sealing portion 125 as shown by the arrow D9. Even in a casewhere the stem-part heat-barrier layer 722 is exposed to the hightemperature, the heat is made to escape to the coolant sealing portion125. Accordingly, the excessive temperature increase of the exhaustvalve 12C which may cause the preignition can be prevented properly.

Operations/Effects

According to the above-described combustion-chamber structure of theengine of the present embodiments, at least the valve heads 113, 123 arecovered with the heat-barrier layer 72 having the smaller heatconductivity than the intake valve body 110, the exhaust valve body 120,and the heat-insulation layer 71. Accordingly, the temperaturedifference between the valve heads 113, 123 and the combustion gas inthe combustion chamber 6 is made so small that the heat transfer to thevalve heads 113, 123 can be suppressed. Further, the heat which haspassed through the heat-barrier layer 72 is stored at theheat-insulation layer 71. Accordingly, the high temperature of theheat-barrier layer 72 (valve heads 113, 123) can be maintained. Thus,the heat loss through the intake valve 11 and the exhaust valve 12 canbe reduced properly.

Meanwhile, the heat-diffusion layer 73 is provided between theheat-insulation layer 71 and the heat-barrier layer 72. Theheat-diffusion layer 73 has the larger heat conductivity than both theheat-barrier layer 72 and the heat-insulation layer 71 and comprises thecontact portion 731 contacting with the valve seat 4S of the cylinderhead 4. Accordingly, even in a case where the heat-insulation layer 71has stored the heat excessively, this heat can be made to escape to thecylinder head 4 through the heat-diffusion layer 73. Consequently, thetemperature increase of the intake valve 11 and the exhaust valve 12which may cause the preignition can be prevented properly.

What is claimed is:
 1. A combustion-chamber structure of an engine,comprising: a cylinder block; a cylinder head; a piston; a valve; and acombustion chamber partitioned by the cylinder block, the cylinder head,the piston, and the valve, wherein said valve is configured to open orclose a port opening to said combustion chamber and comprises anumbrella part and a stem part, said umbrella part of the valve includesa valve body which includes a valve head facing the combustion chamberand a valve face positioned on an opposite side to the combustionchamber, a heat-insulation layer which is provided at the valve head andhas smaller heat conductivity than said valve body, a heat-barrier layerwhich is provided to cover said valve head provided with saidheat-insulation layer and has smaller heat conductivity than said valvebody and said heat-insulation layer, and a heat-diffusion layer which isprovided between said heat-insulation layer and said heat-barrier layerand has larger heat conductivity than said heat-insulation layer andsaid heat-barrier layer, and said heat-diffusion layer comprises acontact portion which is provided to extend up a position of saidumbrella part of the valve which contacts with said cylinder head whenthe valve is closed.
 2. The combustion-chamber structure of the engineof claim 1, wherein said cylinder head has the larger heat conductivitythan said valve body.
 3. The combustion-chamber structure of the engineof claim 2, wherein said cylinder head comprises a valve seat which isprovided at said port opening and with which a portion of said umbrellapart of the valve body contacts, and said contact portion of theheat-diffusion layer is provided at said portion of the umbrella partwhich contacts with the valve seat.
 4. The combustion-chamber structureof the engine of claim 3, wherein said valve is an intake valve, andsaid heat-barrier layer is provided on said valve face of the umbrellapart of the valve as well.
 5. The combustion-chamber structure of theengine of claim 4, wherein said heat-barrier layer is provided on saidstem part of the valve as well.
 6. The combustion-chamber structure ofthe engine of claim 3, wherein said valve is an exhaust valve, and saidheat-barrier layer is provided on said valve face of the umbrella partof the valve as well.
 7. The combustion-chamber structure of the engineof claim 6, wherein said heat-barrier layer is provided on said stempart of the valve as well.
 8. The combustion-chamber structure of theengine of claim 5, wherein said heat-diffusion layer includes a firstportion which is provided between said heat-insulation layer and saidheat-barrier layer at said valve head, said contact portion, and asecond portion which is an underlayer of the heat-diffusion layer whichis provided at said valve face and said stem part.
 9. Thecombustion-chamber structure of the engine of claim 3, wherein saidvalve is an exhaust valve, said heat-insulation layer and saidheat-diffusion layer are provided to cover an entire part of saidumbrella part of the valve, and said heat-barrier layer is provided tocover the entire part of the umbrella part of the valve except saidcontact portion of the heat-diffusion layer.
 10. The combustion-chamberstructure of the engine of claim 9, wherein said heat-insulation layer,said heat-diffusion layer, and said het-barrier layer are provided tocover at least a section of said stem part of the valve which iscontinuous to said umbrella part of the valve.
 11. Thecombustion-chamber structure of the engine of claim 3, wherein saidvalve is an exhaust valve with cooling function in which a coolantsealing portion is formed at the valve body, said heat-insulation layerand said heat-diffusion layer are provided to cover said umbrella partof the valve, said heat-barrier layer is provided to cover the umbrellapart of the valve except said contact portion of the heat-diffusionlayer, and said heat-insulation layer and said heat-diffusion layer areprovided to extend up to a position which overlaps with said coolantsealing portion of the valve body.
 12. The combustion-chamber structureof the engine of claim 1, wherein said cylinder head comprises a valveseat which is provided at said port opening and with which a portion ofsaid umbrella part of the valve body contacts, and said contact portionof the heat-diffusion layer is provided at said portion of the umbrellapart which contacts with the valve seat.
 13. The combustion-chamberstructure of the engine of claim 1, wherein said valve is an intakevalve, and said heat-barrier layer is provided on said valve face of theumbrella part of the valve as well.
 14. The combustion-chamber structureof the engine of claim 1, wherein said valve is an exhaust valve, andsaid heat-barrier layer is provided on said valve face of the umbrellapart of the valve as well.
 15. The combustion-chamber structure of theengine of claim 1, wherein said valve is an exhaust valve, saidheat-insulation layer and said heat-diffusion layer are provided tocover an entire part of said umbrella part of the valve, and saidheat-barrier layer is provided to cover the entire part of the umbrellapart of the valve except said contact portion of the heat-diffusionlayer.
 16. The combustion-chamber structure of the engine of claim 1,wherein said valve is an exhaust valve with cooling function in which acoolant sealing portion is formed at the valve body, saidheat-insulation layer and said heat-diffusion layer are provided tocover said umbrella part of the valve, said heat-barrier layer isprovided to cover the umbrella part of the valve except said contactportion of the heat-diffusion layer, and said heat-insulation layer andsaid heat-diffusion layer are provided to extend up to a position whichoverlaps with said coolant sealing portion of the valve body.
 17. Thecombustion-chamber structure of the engine of claim 12, wherein saidvalve is an intake valve, and said heat-barrier layer is provided onsaid valve face of the umbrella part of the valve as well.
 18. Thecombustion-chamber structure of the engine of claim 13, wherein saidheat-barrier layer is provided on said stem part of the valve as well.19. The combustion-chamber structure of the engine of claim 14, whereinsaid heat-barrier layer is provided on said stem part of the valve aswell.
 20. The combustion-chamber structure of the engine of claim 1,wherein said heat-barrier layer is made of heat-resistant silicon resinwhich has the heat conductivity of 0.05-1.50 W/mK, and saidheat-diffusion layer is made of copper-based material, Corson alloy,beryllium copper, fiber-reinforced aluminum alloy, or titanium aluminumwhich have the heat conductivity of 35-600 W/mK.